go/types: make Identical(nil, T) == Identical(T, nil)

Fixes #15173

Change-Id: I353756f7bc36db0d2b24d40c80771481b7b18f6b
Reviewed-on: https://go-review.googlesource.com/21585Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/go/types/api_test.go

@@ -1042,3 +1042,20 @@ func f() {
 		}
 	}
 }
+
+func TestIdentical_issue15173(t *testing.T) {
+	// Identical should allow nil arguments and be symmetric.
+	for _, test := range []struct {
+		x, y Type
+		want bool
+	}{
+		{Typ[Int], Typ[Int], true},
+		{Typ[Int], nil, false},
+		{nil, Typ[Int], false},
+		{nil, nil, true},
+	} {
+		if got := Identical(test.x, test.y); got != test.want {
+			t.Errorf("Identical(%v, %v) = %t", test.x, test.y, got)
+		}
+	}
+}

src/go/types/predicates.go

@@ -277,6 +277,8 @@ func identical(x, y Type, p *ifacePair) bool {
 			return x.obj == y.obj
 		}
 
+	case nil:
+
 	default:
 		unreachable()
 	}